The present invention relates to a device for and a method of determining the angle of incidence of a received signal by performing a vertical scanning operation using a generally cylindrical transducer, as well as to a scanning sonar employing such device and/or method.
Scanning sonars are widely used for detecting underwater objects. A scanning sonar employs a transducer having a generally cylindrical shape as shown in FIG. 3 to detect underwater objects in all surrounding areas. Ultrasonic waves are radiated in all directions around the transducer and a vertically focused receiving beam, which is focused within a specified angle in a vertical plane to produce specific vertical directivity, is formed by summing signals reflected by the underwater objects and received by every vertical array, or column, of transducer elements with appropriate time delays inserted into the individual reflected signals. The receiving beam thus formed is rotated, as if a single column of transducer elements is mechanically rotated around the cylindrical shape, by switching the transducer elements from one vertical array to another around the transducer to scan the objects in all azimuthal directions. It is possible to control the downward-looking angle, or the tilt angle, of the receiving beam by varying the time delays applied to the individual transducer elements, whereby the scanning sonar can search out objects within a specific vertical sector area in all directions around the transducer.
In the conventional scanning sonar of the aforementioned construction, the receiving beam directed to a specific tilt angle is horizontally rotated around the transducer in each horizontal scanning cycle. Thus, if it is desired to detect vertically distributed objects, it is necessary to carry out successive horizontal scanning cycles while sequentially varying the tilt angle of the receiving beam in one scanning cycle after another.
Although the prior art discloses a transducer constructed in a spherical shape which can steer a receiving beam not only horizontally but also vertically, the spherically shaped transducer has such problems that it necessitates complex combinations of transducer elements for beam forming and its target detecting ability in horizontal scan mode, which is the most often used mode of operation, is inferior to that of the cylindrical transducer.